Servo-control unit for a variable prestressed load

ABSTRACT

The invention relates a servo-control unit for a load ( 2 ) driven in rotation by motorization means ( 1 ) connected to the Load by two reduction gears ( 3, 4 ) and comprising several sensors for measuring their movements It includes a prestressing motoring system ( 5 ) arranged between the two reduction gears ( 3, 4 ) so as to apply a positive and respectively a negative torque to each of the reduction gears. The prestressing system ( 5 ) is designed to apply a torque at any particular point of the operating field of the reduction gears and to introduce therein a variable prestressing level. The prestressing system ( 5 ) is a jack ( 14 ) driven by a motor ( 17 ) and co-operating with a rack or a screw-nut system ( 15 ). The invention is useful for motorization of a turret mounted on a vehicle.

[0001] The technical scope of the present invention is that of devicesto drive a load with reduced or no backlash between the motor and theload.

[0002] When a load is driven in movement, and notably in rotation, thebacklash between said load and the drive motor must be reduced to aminimum so as to obtain the precise positioning of the load with respectto a given reference. This problem is of extreme importance within thescope of the laying of a weapon mounted in the turret of an armedvehicle.

[0003] Various principles to eliminate any backlash are known and usedin various branches of the technique.

[0004] By way of illustration, patent FR-2221981 proposes twoindependent and slightly opposing chains to eliminate the problems ofbacklash, but using two motors. This is the main drawback to thisdevice.

[0005] Patent FR-2538136 proposes to release the servo-driven chain fromthe elasticity of the transmission so that the motorisation iscomparable to that of a torque motor connected directly to the load. Inthe proposed system, the motorisation is constituted by a motorconnected to the load by two reduction gears whose elasticity isemployed to L eliminate the backlash in a transmission by reducedprestressing. This reduced prestressing is enough to absorb thedifferent homokinetic type errors that may appear between motor andload. This system is perfectly well adapted when the ratio between theelasticity of the reduction gears and the maximal homokinetism error ishigh.

[0006] However, when the harmonisation errors exceed around 10% of theclearance between the motor and the load required to transmit themaximum motorisation torque, the principle of approximately fixedprestressed load is used to avoid the arcing of two chains set inparallel, locally or globally over the full kinematics. The inventionaims to supply a device having this property.

[0007] The invention thus relates to a servo-control unit for a loaddriven in rotation by means of motorisation connected to the load by tworeduction gears and comprising sensors for measuring their movements,wherein it comprises a prestressing motoring system arranged between thetwo reduction gears so as to respectively apply a positive and anegative torque to each of the reduction gear.

[0008] Advantageously, the prestressing system is designed to firstlyapply to a torque at any point of the operating field of the reductiongears and secondly to introduce therein a variable prestressing level.

[0009] According to a first embodiment, the prestressing system is ajack driven by a motor and co-operating with a rack or a screw-nutsystem.

[0010] Advantageously, the prestressing system comprises a power stagebox having an epicyclic gear train.

[0011] According to another embodiment, the device comprisesprestressing inputs on the crown of the reduction gear with prestressingmotorisation provided by hydraulic jack and pressure servo.

[0012] According to yet another embodiment, the prestressing system is adouble reduction chain assembly integrating a jack and at least oneinput pinion whose axis of rotation has a degree of freedom with respectto said assembly.

[0013] Advantageously, the pinion is connected to the output of saidassembly by an Oldham coupling type system.

[0014] Advantageously again, the device comprises a connecting rodhinged so as to ensure the radial displacement of the pinion.

[0015] According to yet another embodiment, one of the reduction gearshas a slant toothed gear stage.

[0016] Advantageously, the pinion of the stage has an axial degree offreedom with respect to the wheel, said degree of freedom being used tomodify the prestressing of the two chains by a back-geared motor.

[0017] According to yet another embodiment, the prestressing meanscomprise a jack allowing the reduction gears to be applied to the load.

[0018] Advantageously, the prestressing motorisation system incorporateselectrical control means.

[0019] Additionally, the principle of dual motorisation may be applied,both chains of which are respectively used to supply the positive torqueand negative torque to be transmitted to the load. In this case, theprestressing is made actively by specific control of the motorisationsof each of the chains, thereby ensuring the transmission of positive ornegative torques to the load without having to pass through the backlashof the transmission chains.

[0020] The invention applies in particular to systems having a lot ofbacklash between the motor and the load, such backlash being nonnegligible in relation to the angular strain of the transmission underthe maximum stress to be transmitted. The second particularity lies inthat the system is particularly sensitive to friction torques in thetransmission both from a final accuracy but also an energetic point ofview.

[0021] Other characteristics, particulars and advantages of theinvention will become more apparent from the following description,given by way of illustration and in relation to the appended drawings,in which:

[0022]FIG. 1 is a general schematic illustration of the device accordingto the invention,

[0023]FIG. 2 is a general block diagram of the device according to theinvention,

[0024]FIG. 3 shows a first embodiment of the invention,

[0025]FIG. 4 shows an epicyclical train,

[0026]FIG. 5 shows a second embodiment of the invention,

[0027]FIG. 6 shows a third embodiment of the invention, and

[0028]FIG. 7 is a section view of a fourth embodiment of the invention.

[0029]FIG. 1 shows that the device according to the inventionschematically comprises a motor 1 dimensioned so as to supply a load 2with torque and velocity. Between the motor 1 and the load 2 a doublereduction chain is placed which comprises reduction gears 3 and 4 toconnect the load to the motor. Prestressing motorisation means 5 areplaced between the reduction gears 3 and 4, each provided with an inputin a position used by the prestressing motorisation means. Naturally,this control allows a desired level of prestressing to be introduction.Lastly, a specific means 6 allow the controls made by the means 5 to bedefined.

[0030] The structure thus described allows the reduction chains to beadapted to the different natural anomalies of the transmission system,which with zero torque transmitted in each of the chains, corresponds tothe relative position between the motor 1 and the load 2 in which may beincluded:

[0031] the influence of the temperature variation on the differentialdilations between the gear housing and the internal mechanical elements.

[0032] the ovalization errors of a toothed or circular sector,

[0033] the homokinetism errors between the motor and the load, due toerrors in the tooth formation of the gears.

[0034] According to the mechanical reduction principle used, the systemto add the prestressing may take various forms. These will be classifiedinto two families respectively corresponding:

[0035] to all the electromechanical systems allowing an additionalangular displacement to be added in each of the chains. The fact inparticular that there is little clearance to be added is exploited. Forexample, this limited displacement may be obtained by a radial relativedisplacement between the output pinions and the sector or circular gearto be driven. The potential degree of freedom of a reduction stage byepicyclic train may also be used to introduce the required variable.Another variant lies in the use of the axial displacement of a slanttoothed reduction pinion to recover the required degree of freedom,

[0036] to using a hydraulic jack type system with pressure servo, whichconstitutes a well adapted means to ensure the contact, of both chainswith no backlash and with controlled arcing.

[0037] The nominal advantage of the variable prestressing motorisationsystem lies in that it constitutes a device that firstly compensates forthe differential homokinetic variations between the two reduction chainsand secondly increases the prestressing of the torque to be transmittedduring transmission. This optimisation of the prestressing torque infact allows the friction torque in the motorisation chain to beminimised and thus its corresponding effect firstly upon the accuracy ofthe load's servo system and secondly on the energetic cost of themotorisation control. In fact, fixed prestressing systems allowing anapproximately constant rigidity of the motorisation chain to be ensuredin all the range of torques to be transmitted and in that of thepositions of the load directly result in the increase in the level ofthe friction torque to be overcome with the dual disadvantage on theaccuracy of the load's servo system and on the energetic cost of thecontrol.

[0038] The means proposed to vary the prestressing, using the control,in the motorisation chain must therefore be associated with adaptedcontrols. In these controls will be differentiated firstly the principleallowing the instantaneous torque to be applied in the prestressingchain to be associated with the control principle allowing thisprestressing to be made on the secondary chain.

[0039] The processing to be made on the signal to be picked up from themain chain is a conventional processing allowing a prestressing torquevalue corresponding to an absolute value of the maximum torque to bedetermined. Principles of the double alternation filtering type mayusefully be applied. As for the prestressing chain torquing control, anymeans, either to measure the prestressing torque set in the transmissionsystem or to identify the requirement in angular position of theprestressing motor corresponding respectively to the homokinetism errorsto be overcome and to the additional angle corresponding to theprestressing torque to be established, may be employed.

[0040]FIG. 2 shows a general block diagram illustrating, firstly, themain motorisation chain 4 organised so as to allow a prestressedadjustment input to be introduced into the reduction chain of thecontrol of the main chain 7, and secondly, the principle of generation 8of the variable prestressed torque using the estimation of theinstantaneous torque to be set into the main chain 7 and the control 6principle establishing the prestressing.

[0041] The concept of a back-geared motor having variable prestressingis not specific to a particular gear reducing principle. It is howevernecessary to introduce local modifications allowing at the least aprestressed input into the double chain of the reduction gear. By way ofan example, we quote the following means to perform variableprestressing:

[0042] epicyclic train power stage boxes with prestressed inputs of thecrown gear with motorisation of the prestressing by hydraulic jack andpressure servo-control, or electrical micro-jack controlled in position.

[0043] double-reduction chain box with at least one output gear whoseaxis of rotation has a degree of freedom relative to the box (radial orcircular). The pinion is connected to the box output by an Oldhamcoupling type system and the relative position of its axis in relationto the circular is adjusted by a servo motorisation in position.

[0044] a double chain reduction gear one stage of which is made ofslanted toothed gear. The associated pinions have an axial degree offreedom with respect to the box, this degree of freedom being used tomodify the prestressing in the two chains by a back-geared motor typesystem.

[0045] an oscillating aiming box and circular box adjusting mini-jack.

[0046] eccentric rollers on a rack.

[0047] These embodiments are exemplified as follows.

[0048]FIG. 3 shows a perspective view of an embodiment of theprestressing system 5 integrated into epicyclic train reduction gears 3and 4. Between the motor 1 and the load 2, there are, in order, theinput pinion 9 a or 9 b, the epicyclic gear train 3 or 4, and the outputwheel 10. The epicyclic train 3 or 4 classically comprises an immobilecrown gear 11 with inner toothing, and input pinion 12 and three planetgears 13 meshing on the inner toothing of the crown gear. According tothe invention, a prestressing is introduced by making the crown gear 11mobile in both directions by means of a slight rotation. By slightrotation, we mean a rotation of around 5°. This rotation is obtainedusing a double rack 14 driven in translation by an electricalprestressing jack 15-17. This jack is of a known type and will not bedescribed in further detail.

[0049]FIG. 4 illustrates the classical structure of the epicyclic geartrain where we find the input pinion 12 integral with the drive shaft 18(connected to the motor 1) aligned along the drive axis XX′, thesatellite gears 13 integral with the output shaft 19, also aligned alongaxis XX′. Classically, the satellites 13 mesh on the inner toothing ofthe crown gear 11. This crown 11 is classically immobile with respect toits support and is made mobile in the invention so as to introduce thevariable prestressing as explained previously.

[0050]FIG. 5 shows another embodiment of the variable prestressingmeans. It corresponds to a back-geared motor with a double reductionchain between a motor 1 and the load 2. Each of the two reduction gearsmesh on the inner wheel 10 respectively via pinions 21 and 22. Pinion21, is angularly connected to the box output axis by a coupling 20 of anOldham type. A hinged connecting rod 24 allows the radial displacementof the pinion 21 with respect to the circular 10. The radialdisplacement or pinion 21 causes a relative angular displacement ofpinion 21 with respect to pinion 22, thereby modifying the prestressing.The jack 23 allows this prestressing to be adjusted.

[0051]FIG. 6 shows a section of another embodiment of prestressing meansaccording to which the reduction stage is constituted by slant toothedpinions. Thus, the wheel 26 meshes with pinion 28 via its slantedtoothing. The pinion 28 is driven by the motor 29 by pinions 30 and 31.To adjust the prestressing, pinion 28 is caused to translate, using thepositioning jack 32. The displacement of pinion 28 following arrow Fallows it to be more or less meshed in the toothing of wheel 26 therebycausing the prestressing to vary. The figure does not feature the secondreduction chain, which is quite classical.

[0052]FIG. 7 shows another embodiment of prestressing means. Itcomprises a prestressing jack 35 allowing the reduction gear 3 and 4 tobe applied in an adjustable manner against the toothing of the load 2.The figure shows the main motor 36 and the axis 37 of articulation ofthe back-geared motor. Naturally, the jack 35 is placed between the gearhousing 38 and the support for the load 2.

1. A servo-control unit for a load (2) drive in rotation by motorizationmeans (1) connected to the load by two reduction gears (3, 4) andcomprising sensors to measure their movements, wherein it comprises aprestressing motoring system (5) arranged between the two reductiongears (3, 4) so as to respectively apply a positive an negative torqueto each of the reduction gear.
 2. A servo-control unit according toclaim 1, wherein the prestressing system (5) is able to firstly apply atorque at any point of the operating field of the reduction gears andsecondly introduce into it a variable prestressing level.
 3. Aservo-control unit according to claim 2, wherein the prestressing system(5) is a jack (14) driven by a motor (17) a co-operating with a rack orscrew-nut system (15).
 4. A servo-control unit according to claim 1 or2, wherein the prestressing system (5) comprises a power stage boxhaving an epicyclic gear train (13).
 5. A servo-control unit accordingto claim 4, wherein it comprises prestressing inputs on the crowns (11)of the reduction gear (3, 4) with prestressing motorization provided byhydraulic jack and pressure servo-control.
 6. A servo-control unitaccording to claim 4, wherein the prestressing system (5) is a doublereduction chain (21, 22) assembly integrating a jack (23) and at leastone input pinion (21) whose axis of rotation has a degree of freedomwith respect to said assembly.
 7. A servo-control unit according toclaim 6, wherein the pinion (21) is connected to the output of saidassembly by an Oldham coupling type system (20).
 8. A servo-control unitaccording to claim 7, wherein it comprises a connecting rod (24) hingedso as to ensure the radial displacement of the pinion (21).
 9. Aservo-control unit according to claim 4, wherein one of the reductiongears has a slant toothed gear stage (26, 28).
 10. A servo-control unitaccording to claim 9, wherein the pinion (28) of the stage has an axialdegree of freedom with respect to the wheel (26), said degree of freedombeing used to modify the prestressing of the two chains by a back-gearedmotor (29).
 11. A servo-control unit according to claim 4, wherein theprestressing means comprise a jack (35) allowing the reduction gears tobe applied to the load (2).
 12. A servo-control unit according to claim1, wherein the prestressing motorization system incorporates electricalcontrol means.